Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM.
Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast SEMs, which only image the surface of a material, TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples, also referred to as lamellae, are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. The term “TEM” as used herein refers to a TEM or an STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. The term “S/TEM” as used herein also refers to both TEM and STEM.
Several techniques are known for preparing TEM specimens. These techniques may involve cleaving, chemical polishing, mechanical polishing, or broad beam low energy ion milling, or combining one or more of the above. The disadvantage to these techniques is that they are not site-specific and often require that the starting material be sectioned into smaller and smaller pieces, thereby destroying much of the original sample.
Other techniques generally referred to as “lift-out” techniques use focused ion beams to cut the sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate. Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as materials general to the physical or biological sciences. These techniques can be used to analyze samples in any orientation (e.g., either in cross-section or in plan view). Some techniques extract a sample sufficiently thin for use directly in a TEM; other techniques extract a “chunk” or large sample that requires additional thinning before observation. In addition, these “lift-out” specimens may also be directly analyzed by other analytical tools, other than TEM. Techniques where the sample is extracted from the substrate within the FIB system vacuum chamber are commonly referred to as “in-situ” techniques; sample removal outside the vacuum chamber (as when the entire wafer is transferred to another tool for sample removal) are call “ex situ” techniques.
Samples which are sufficiently thinned prior to extraction are often transferred to and mounted on a metallic grid covered with a thin electron transparent film for viewing. FIG. 13A shows prior art TEM grid 10 for mounting a sample. A typical TEM grid 10 is made of copper, nickel, or gold.
Although dimensions can vary, a typical grid might have, for example, a diameter of 3.05 mm and have a middle portion 12 consisting of cells 14 of size 90 μm by 90 μm and bars 13 with a width of 35 μm. The electrons in an impinging electron beam will be able to pass through the cells 14, but will be blocked by the bars 13. The middle portion 12 is surrounded by an edge portion 16. The width of the edge portion is 0.225 mm. The edge portion 16 has no cells, with the exception of the orientation mark 18. The thickness 15 of the thin electron transparent support film is uniform across the entire sample carrier, with a value of approximately 20 nm. TEM specimens to be analyzed are placed or mounted within cells 14.
In one commonly used ex-situ sample preparation technique, a protective layer 22 of a material such as tungsten is first deposited over the area of interest on a sample surface 21 as shown in FIG. 14 using electron beam or ion beam deposition. Next, as shown in FIGS. 15-16, a focused ion beam using a high beam current with a correspondingly large beam size is used to mill large amounts of material away from the front and back portion of the region of interest. The remaining material between the two milled rectangles 24 and 25 forming a thin vertical sample section 20 that includes an area of interest. The trench 25 milled on the back side of the region of interest is smaller than the front trench 24 The smaller back trench is primarily to save time, but the smaller trench also prevents the finished sample from falling over flat into larger milled trenches which may make it difficult to remove the specimen during the micromanipulation operation.
As shown in FIG. 17, once the specimen reaches the desired thickness, the stage is tilted and a U-shaped cut 26 is made at an angle partially along the perimeter of the sample section 20, leaving the sample hanging by tabs 28 at either side at the top of the sample. The small tabs 28 allow the least amount of material to be milled free after the sample is completely FIB polished, reducing the possibility of redeposition artifacts accumulating on the thin specimen. The sample section is then further thinned using progressively finer beam sizes. Finally, the tabs 28 are cut to completely free the thinned lamella 27. Once the final tabs of material are cut free lamella 27 may be observed to move or fall over slightly in the trench. A completed and separated lamella 27 is shown in FIG. 18.
The wafer containing the completed lamella 27 is then removed from the FIB and placed under an optical microscope equipped with a micromanipulator. A probe attached to the micromanipulator is positioned over the lamella and carefully lowered to contact it. Electrostatic forces and vacuum forces will attract lamella 27 to the probe tip 29 as shown in FIG. 19. The tip 29 with attached lamella is then typically moved to a TEM grid 10 as shown in FIG. 20 and lowered until lamella is placed on the grid in one of the cells 14 between bars 13.
Samples which require additional thinning before observation are typically mounted directly to a TEM sample holder. FIG. 13B shows a typical TEM sample holder 31, which comprises a partly circular 3 mm ring. In some applications, a sample 30 is attached to a finger 32 of the TEM sample holder by ion beam deposition or an adhesive. The sample extends from the finger 32 so that in a TEM (not shown) an electron beam will have a free path through the sample 30 to a detector under the sample. The TEM sample is typically mounted horizontally onto a sample holder in the TEM with the plane of the TEM grid perpendicular to the electron beam, and the sample is observed. A common in-situ extraction technique is described in U.S. Pat. No. 6,570,170, to Moore, which describes extracting out a sample by making a “U”-shaped cut and then cutting the sample at an angle from the missing side of the “U” to undercut and free the sample. After the sample is freed, a probe is attached to the sample by FIB-induced chemical vapor deposition and it is lifted out. In other applications, the probe is attached before the sample is freed. This process typically results in a chunk-type sample, which is generally wedge shaped and approximately 10 μm×5 μm×1 μm in size.
Unfortunately, preparation of TEM samples using such prior art methods of sample extraction suffer from a number of shortcomings. Such methods are typically very time-consuming, about 90 minutes/sample, and labor intensive. CD metrology often requires multiple samples from different locations on a wafer to sufficiently characterize and qualify a specific process. In some circumstances, for example, it will be desirable to analyze from 15 to 50 TEM samples from a given wafer. When so many samples must be extracted and measured, using known methods the total time to process the samples from one wafer can be days or even weeks. Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control.
Specifically, the ex-situ method discussed above can be time consuming and difficult to locate a lamella site and the extraction probe must be very carefully moved into position to avoid damaging the sample or the probe tip. Once a lamella has been completely freed, it can move in unpredictable ways; it can fall over in the trench or in some cases it can actually be pushed up and out of the trench by electrostatic forces. This movement can make it difficult to locate and/or pick up the lamella with the extraction probe. The electrostatic attraction between the probe and the sample is also somewhat unpredictable. In some cases, the lamella may not stay on the probe tip. Instead, it can jump to a different part of the probe. In other cases, the lamella may fall off while the sample is being moved. If the lamella is successfully transferred to the TEM grid, it can be difficult to get the lamella to adhere to the grid support film rather than the probe tip. The lamella will often cling to the probe tip and must be essentially wiped off onto the film. As a result, it can be very difficult to control the precise placement or orientation of the lamella when it is transferred to the TEM grid. The in-situ method described above provides more control but is considerably more time-consuming. A significant amount of time is taken up by the steps of attaching the microprobe to the sample, attaching the sample to the sample holder, and cutting the microprobe free. The sample is also moved to and attached to the TEM grid inside the FIB instrument, which requires more valuable FIB time.
Speeding up the process of sample extraction and transfer would provide significant advantages in both time and potential revenue by allowing a semiconductor wafer to be more rapidly returned to the production line. Full or partial automation of the process of sample removal and transport would not only speed up the process, but it would also reduce the level of expertise required of operators and technicians thus lowering personnel costs.
Because of the precision required to produce thin lamella, the process has not adapted itself to automation. The thinner the lamella, the more difficult it is to automate the extraction process. Lamella under 100 nm in thickness, particularly lamella under 70 nm in thickness, are difficult to produce either manually or in an automated fashion. Slight changes in the positioning of the ion beam can ruin the lamella by producing the lamella at a position that excludes the feature of interest or by altering the thickness. The thin lamella can shift position slightly, which changes the position of the beam relative to the lamella. The thin lamella can also warp during formation, due to mechanical or thermal stress. These factors combine to make the formation of lamella an exceedingly difficult process to automate.
What is needed is an improved method for TEM sample preparation that is robust, repeatable, and can be automated.